1. FIELD OF THE INVENTION
This invention relates generally to the field of weighing scales. More particularly, this invention relates to a method and apparatus for weighing objects on moving platforms such a the deck of a ship wherein movement of the platform can upset the accuracy of the weight measurement.
2. BACKGROUND
In attempting to weigh objects on moving surfaces such as the deck of a ship at sea, the constant rotational accelerations due to the rolling and pitching of the ship can introduce significant errors in the accuracy of conventional weight measuring devices. Such roll and pitch movements can result in accelerations of .+-.1/4 g or even more depending on the vessel type and state of the sea. A 1/4 g acceleration typically produces a weight variation of 25% so that a window of .+-.25% on the accuracy of a conventional weight measurement is possible.
This is clearly an unacceptable variation in the accuracy of weight determination by almost any standard. It can be particularly troublesome to fisherman who need a determination of the weight of a catch at sea or to scientists who must make weight measurements at sea.
FIG. 1 and FIG. 2 illustrate in schematic form the conventional weight measurement devices of the prior art. In FIG. 1, the conventional spring scale is shown. In this device, the actual weight of the object 10 is measured. Object 10 is placed in a scale pan 12 which is attached to one end of a spring 14. The scale pan 12 is also attached to a pointer 16 which points to a graduated scale 18. The other end of the spring is held rigid and the deformation of the spring is correlated to a weight of the object which may be read out on the graduated scale 18. Unfortunately, this scale depends upon no outside accelerations being applied to the rigid end 20. Such accelerations are difficult or impossible to avoid on a ship at sea.
FIG. 2 shows the other conventional mechanism for weight measurement--the balance. In this mechanism, the object 10 of unknown weight is compared with an object of known weight 24. The balance, therefore, actually measures mass which is related to weight by the acceleration of gravity: EQU F=m*A (1)
where:
F=weight PA1 m=mass PA1 A=the acceleration of gravity. PA1 .theta.1 is the angle between F1 and f1, PA1 .theta.2 is the angle between F2 and f2.
The variations in the effect of gravity will be ignored for the purposes of this discussion and the terms weight and mass may be used interchangeably.
The conventional balance uses a pair of scale pans 26 and 28 which are used to carry the unknown weight 10 and the known weight 24 respectively. These pans are suspended from the ends of a beam 30 which is rigidly balanced in the middle as shown at 32. Of course, it need not be suspended in the middle if this is taken into consideration in the weight comparison.
If it is assumed that the acceleration of gravity is constant, the balance can then be used to measure weight. Unfortunately, the balance is also prone to disruption in accuracy by the roll and pitch of a ship at sea. Consider a model of this movement as an angular acceleration R about point 34 as shown. The centrifugal forces F1 and F2 as shown are given by: EQU F1=r1*R*m (2) EQU F2=r2*R*m (3)
The components contributing to the unbalance are: EQU f1=F1*cos (.theta.1) (4) EQU f2=F2*cos (.theta.2) (5)
where
Thus, except for the rare instances where .theta.1 and .theta.2 are equal, the balance also does not provide an accurate measurement when subjected to outside accelerations.
Most conventional weight measurement devices use one of these two principles to measure weight. Some use electronic pressure transducers and the like as a substitute for a spring and operate under computer control.
The problem of measuring weight at sea or in the presence of outside accelerations is largely alleviated by the present invention which provides a novel arrangement for a weight measurement apparatus.